Acrylic-based thermally conductive composition and thermally conductive sheet

ABSTRACT

Provided is a thermally conductive composition which shows a low thermal resistance, and good releasability after service. For example, one composition includes an acrylic-based thermally conductive composition comprising a binder component containing a crystalline acrylic polymer with an alkyl group of 18 carbons or more and a thermally conductive filler.

TECHNICAL FIELD

The present invention relates to an acrylic-based thermally conductivecomposition and to a thermally conductive sheet.

BACKGROUND

Radiating release of heat generated by various electronic and electricaldevices such as personal computers is a matter of major importance.Thermally conductive compositions and thermally conductive sheetsobtained therefrom are used as heat radiating means to allow heat fromheat generating parts of electronic and electric devices to escape toheat radiating parts such as heat sinks and metal covers. They are alsoused for anchoring between electronic parts and heat radiating parts.

Because the thermally conductive sheet is used while being sandwichedbetween a heat generating part and a heat radiating part, the contactsat the interfaces between the sheet and those parts are important inview of the thermal conductivity. That is, when the contacts at theinterfaces are not good, the thermal resistance between the parts andthe sheet increases and thermal conductivity becomes lower. Accordingly,the heat conductive sheet must be contacted with a heat generating partand a heat radiating part while the sheet is conforming to fineirregularities present on the surfaces of the parts. The thermallyconductive sheet is therefore required to have flexibility and adhesionto the heat generating part and the heat radiating part. However, athermally conductive sheet with high adhesion generally shows poorhandling and is difficult to be released after service.

Japanese Unexamined Patent Publication (Kokai) No. 2000-336279 disclosesa thermally conductive composition and a thermally conductive sheet inwhich wax such as paraffin wax or softening agents, tackifiers andthermally conductive, fillers are added to a thermoplastic resin.Addition of wax to the composition improves adhesion to a body to whichthe sheet is applied. However, release of the sheet from the bodybecomes difficult after service. Oil is specifically used as a softeningagent. However, volatilization of the oil decreases the flexibility ofthe composition and adhesion of the sheet to the body, or may causecontamination of electronic parts.

Japanese National Patent Publication (Kohyo) No. 2000-509209 discloses athermally conductive composition comprising an acrylicpressure-sensitive adhesive component, an α-olefin thermoplasticcomponent, paraffin wax and thermally conductive fillers. Because wax isused, thermally conductive sheets formed from the composition aredifficult to be released after service similarly to the compositiondescribed in Patent Publication 336279 mentioned above. Although thecomposition contains an α-olefin thermoplastic component and an acrylicpressure-sensitive adhesive component, adhesion of the sheets to a bodyto which the sheets are applied becomes insufficient due to aninsignificant decrease in viscosity during heating, and the thermalconductivity is lowered.

Japanese Unexamined Patent Publication (Kokai) No. 2001-89756 disclosesthermally intermediating materials comprising polymer components,melting point components such as wax or wax-like compounds and thermallyconductive fillers. Thermally conductive sheets formed from thesecompositions are difficult to be released after service similarly to thecomposition described in Patent Publication 336279. Moreover, themelting point components and polymer components must be selected whilethe compatibility of these components are taken into consideration, andtherefore a flexibility in selections of materials are restricted.

Specification of U.S. Pat. No. 6,399,209 discloses a thermallyconductive pad containing a laminate that comprises a flexible layercontaining polysiloxane wax having alkyl group substituents andconductive fillers and an anti-blocking layer. A polyester having aglass transition temperature of 70° C. or higher is used as theanti-blocking layer. As a result, the pad is excellent in releasabilityafter service; however, the thermal resistance at the interface isincreased due to the high melt viscosity.

Japanese Unexamined Patent Publication (Kokai) No. 2002-234952 disclosesheat-release sheets composed of a composition containing a polymer gel(A), a compound (B) that is solid or paste at room temperature andbecomes liquid when heated and a thermally conductive filler (C).Specifically, examples of the compound (B) include silicone oils, waxand α-olefins. For such materials, handling of the sheets duringreleasing is poor, and the thermal conductivity of the sheets are notgood because adhesion of the sheets to bodies to which the sheets areapplied is decreased due to volatilization of the materials duringservice of the sheets; moreover, the materials cause contamination ofthe bodies.

SUMMARY

It is therefore an object of the present invention to provide athermally conductive composition and a thermally conductive sheet havinglow thermal resistances and showing good releasability after service.

According to one embodiment of the present invention, there is providedan acrylic-based thermally conductive composition comprising a bindercomponent containing a crystalline acrylic polymer with an alkyl groupof 18 carbons or more and a thermally conductive filler.

Such a thermally conductive composition has high adhesion to a body withwhich the composition is contacted during service to decrease thethermal resistance, and as a result the composition has high thermalconductivity. However, the composition shows good releasability afterservice.

In addition, the term “crystalline acrylic polymer” used in the presentspecification means an acrylic polymer that shows a melting peak whenmeasured by differential scanning calorimetry (DSC) while heated at arate of 5° C./min and that is obtained from monomers containing anacrylic and/or methacrylic monomer.

Moreover, the term “crystalline acrylic monomer” means that thehomopolymer of which becomes a crystalline acrylic polymer.

The term “noncrystalline acrylic polymer” means an acrylic polymer otherthan a crystalline acrylic polymer.

The term “noncrystalline acrylic monomer” means that the homopolymer ofwhich becomes a noncrystalline acrylic polymer.

DETAILS OF PRESENTLY PREFERRED EMBODIMENTS

Binder Component

A crystalline acrylic polymer is used as the binder component of thethermally conductive composition of the present invention. Thecrystalline acrylic polymer composing the binder component thereforeshows a melting point. The binder component is melted at temperaturehigher than the melting point of the crystalline acrylic polymer toimprove adhesion to a body with which the composition is contacted. As aresult, the heat resistance at the interface between the body and thecomposition is lowered, and the composition exhibits high thermalconductivity. On the other hand, when the composition after service iscooled to temperature lower than the melting point of the crystallineacrylic polymer, the crystalline acrylic polymer is solidified tofacilitate release of the sheet. Accordingly, the crystalline acrylicpolymer contained in the binder component should be made to have amelting point lower than the temperatures to which the polymer isexposed during the service of the thermally conductive composition. Thecrystalline acrylic polymer preferably has a melting point higher thanroom temperature (e.g., 25° C. or higher) and 100° C. or lower, morepreferably 30° C. or higher and 60° C. or lower.

The crystalline acrylic polymer is a polymer of a crystalline acrylicmonomer, and may be either a homopolymer or copolymer. The crystallineacrylic monomer includes a (meth)acrylate ester monomer with an alkylgroup of 18 carbons or more. Examples of the crystalline acrylic monomerinclude octadecyl (meth)acrylate, (also termed “stearyl(meth)acrylate”), nonadecyl (meth)acrylate, icosanyl (meth)acrylate,henicosanyl (meth)acrylate, docosanyl (meth)acrylate (also termed“behenyl (meth)acrylate”), tricosanyl (meth)acrylate, tetracosanyl(meth)acrylate and octyldodecyl (meth)acrylate. The crystalline acrylicmonomer is obtained by, for example, polymerizing one of the abovemonomers, or a mixture of two monomers or more of the above monomers.Moreover, the crystalline acrylic polymer can also be obtained from acrystalline macromonomer that is synthesized from one of the abovemonomers. Such a crystalline macromonomer is an oligomer prepared from acrystalline acrylic monomer and having a polymerizing functional groupsuch as a terminal (meth)acryloyl group. Specifically, an octadecyl(meth)acrylate oligomer having a (meth)acryloyl terminal group can bementioned as the crystalline macromonomer.

Furthermore, the crystalline acrylic polymer may also be a copolymer ofa crystalline acrylic monomer and a noncrystalline acrylic monomer. Forexample, the crystalline acrylic polymer is a copolymer of a crystalline(meth)acrylate ester monomer with an alkyl group of 18 carbons or moreand a noncrystalline acrylic monomer. The noncrystalline acrylic monomeris, for example, a (meth)acrylic monomer with an alkyl group of 12carbons or less. In more detail, examples of the noncrystalline acrylicmonomer include ethyl (meth)acrylate, butyl (meth)acrylate, hexyl(meth)acrylate, 2-ethylhexyl (meth)acrylate, octyl (meth)acrylate,isooctyl (meth)acrylate, decyl (meth)acrylate and dodecyl(meth)acrylate. Moreover, to increase cohesion of the resultingthermally conductive composition, a (meth)acrylic monomer with ahomopolymer glass transition temperature of 20° C. or higher may also beused additionally. As such monomers there may be mentioned carboxylacids and their corresponding anhydrides, such as acrylic acid and itsanhydride, methacrylic acid and its anhydride, itaconic acid and itsanhydride, and maleic acid and its anhydride. Other examples of(meth)acrylic monomers with homopolymer glass transition temperatures of20° C. or higher include cyanoalkyl (meth)acrylates, acrylamide,substituted acrylamides such as N,N′-dimethylacrylamide, and polarnitrogen-containing materials such as N-vinylpyrrolidone,N-vinylcaprolactam, N-vinylpiperidine and acrylonitrile. Other monomersinclude tricyclodecyl (meth)acrylate, isobornyl (meth)acrylate, hydroxy(meth)acrylate and vinyl chloride. In addition, when the crystallineacrylic polymer is a copolymer of a crystalline acrylic monomer and anoncrystalline acrylic monomer, the crystalline acrylic polymer is acopolymer prepared from a monomer mixture containing preferably 30% byweight or more, more preferably 50% by weight or more of a crystallineacrylic monomer based on the monomer weight so that the crystallineacrylic polymer has sufficient crystallinity.

The binder component may include other polymer components other than thecrystalline acrylic polymer as long as the effects of the presentinvention are not marred. Examples of the other polymer componentsinclude a noncrystalline acrylic polymer. The binder component in thiscase is a mixture of a crystalline acrylic polymer and a noncrystallineacrylic polymer. The noncrystalline acrylic polymer is obtained bypolymerizing the above noncrystalline acrylic monomer. In order for thecomposition to have a high adhesion to a body with which the compositionis contacted during service and to show sufficient releasability duringcooling the body after service, the binder component preferably contains10% by weight or more, more preferably 30% by weight or more, and stillmore preferably 50% by weight or more of the crystalline acrylic polymerbased on the weight of the binder component.

Thermally Conductive Filler

The acrylic-based thermally conductive composition of the presentinvention contains a thermally conductive filler. As thermallyconductive fillers there may be used ceramics, metal oxides, metalhydroxides, metal and the like. Specifically, there may be mentionedthermally conductive fillers such as aluminum oxide, silicon oxide,magnesium oxide, zinc oxide, titanium oxide, zirconium oxide, ironoxides, aluminum hydroxide, silicon carbide, boron nitride, aluminumnitride, titanium nitride, silicon nitride, titanium boride, carbonblack, carbon fiber, carbon nanotube, diamond, nickel, copper, aluminum,titanium, gold and silver. Crystalline form thereof can be anycrystalline form that can be formed by the respective chemical species,such as hexagonal or cubic system. The filler particle size will usuallybe 500 μm or smaller. If the filler particle size is too large, thesheet strength is reduced. It is preferred to combine a portion oflarge-sized particles with a portion of small-sized particles. This isbecause the small particle size portion will fill the space between thelarge particle size portions, thus increasing the amount of filler thatcan be loaded. The particle size of the large particle size portion ispreferably 10-150 μm, and the particle size of the small particle sizeportion is below that of the large particle size portion, and preferablyless than 10 μm. For improved sheet strength, a filler which has beensurface-treated with silane, titanate or the like may be used. Inaddition, a filler coated thereon with a coating such as water resistantor insulative coat made of ceramics or polymers may be used. Inaddition, the term “particle size” used here means the dimension of thelongest length as measured in a straight line passing through the centerof gravity of the filler. The filler shape may be regular or irregular,for example, polygonal, cubic, elliptical, spherical, needle-like,plate-like or flaky, or combination of these shapes. Further, a fillercan be a particle formed by aggregating crystalline particles. Thefiller shape may be selected based on the viscosity of the thermallypolymerizing binder component and ease of working the final, polymerizedthermally conductive composition.

Further, an electromagnetic wave absorbing filler can be added in orderto impart an electromagnetic wave absorbing property. An electromagneticwave absorbing filler includes soft ferrite compounds such as Ni—Znferrite, Mg—Zn ferrite and Mn—Zn ferrite, magnetically soft metals suchas carbonyl iron and Fe—Si—Al alloy (sendust), and carbon. Since anelectromagnetic wave absorbing filler itself is also thermallyconductive, an electromagnetic wave absorbing filler may be used alone,or as a mixture with a thermally conductive filler.

Other Additives

The thermally conductive composition of the invention may also containadditives such as tackifiers, antioxidants, plasticizers, flameretardants, anti-settling agents, thickeners such as acryl rubber andepichlorohydrin rubber, thixotropic agents such as micronized silicapowder, surfactants, antifoaming agents, coloring agents, electricconductive particles, antistatic agents, organic fine particles, ceramicbubbles and the like. Alternatively, combinations of the foregoingadditives may also be used.

Production of Acrylic-Based Thermally Conductive Composition andThermally Conductive Sheet

The acrylic-based thermally conductive composition of the presentinvention can be produced by mixing a (meth)acrylic monomer (includingcrystalline acrylic monomer) for a polymer composing the bindercomponent and a thermally conductive filler, and then carrying outpolymerization. However, because the viscosity of the (meth)acrylicmonomer will generally be low without further processing, the filler maysettle when a thermally conductive filler is mixed with the(meth)acrylic monomer-containing binder component prior topolymerization. In such cases, it is preferred to partially polymerizethe (meth)acrylic monomer beforehand to increase the viscosity. Suchpartial polymerization is preferably carried out to a viscosity of about100-10,000 centipoise (cP) in terms of the thermal polymerizing bindercomponent. The partial polymerization may be carried out by any ofvarious methods such as, for example, thermal polymerization,ultraviolet polymerization, electron beam polymerization, γ-raypolymerization and ionizing irradiation.

A thermal polymerization initiator or photopolymerization initiator iscommonly used for such partial polymerization. As thermal polymerizationinitiators there may be used organic peroxide free radical initiatorssuch as diacyl peroxides, peroxyketals, ketone peroxides,hydroperoxides, dialkyl peroxides, peroxy esters, peroxy dicarbonatesand the like. Specifically there may be mentioned lauroyl peroxide,benzoyl peroxide, cyclohexanone peroxide,1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, t-butylhydroperoxide,bis(4-t-butylcyclohexyl)peroxydicarbonate,1,1-bis(t-hexylperoxy)-3,3,5-tricyclohexane,1,1-bis(t-hexylperoxy)cyclohexane, 1,1-bis(t-butylperoxy)cyclohexane,and the like. Alternatively, persulfate/bisulfite combinations may alsobe used.

As photopolymerization initiators there may be mentioned benzoin etherssuch as benzoin ethyl ether or benzoin isopropyl ether, anisoin ethylether and anisoin isopropyl ether, Michler's ketone(4,4′-tetramethyldiaminobenzophenone), or substituted acetophenones suchas 2,2-dimethoxy-2-phenylacetophenone (for example, KB-1 by Sartomer;Irgacure™ 651 by Ciba-Specialty Chemical) and 2,2-diethoxyacetophenone.In addition there may be mentioned substituted α-ketols such as2-methyl-2-hydroxypropiophenone, aromatic sulfonyl chlorides such as2-naphthalenesulfonyl chloride, and photoactive oxime-based compoundssuch as 1-phenone-1,1-propanedione-2-(o-ethoxycarbonyl)oxime. Anycombination of the foregoing thermal polymerization initiators orphotopolymerization initiators may also be used.

The amount of the initiator used for partial polymerization is notparticularly restricted, but will normally be 0.001-5 parts by weight to100 parts by weight of the (meth)acrylic monomer.

Furthermore, for the partial polymerization, the partial polymerizationcan be carried out with a chain transfer agent to control the molecularweight and the content of the polymer included in the obtained partiallypolymerized polymer. Examples of the chain transfer agent includemercaptans, disulfides, carbon tetrabromide, carbon tetrachloride or thelike, and combinations thereof. If used, the transfer agent is generallyused in an amount of 0.01-1.0 part by weight based on 100 parts byweight of the (meth)acrylic monomer.

A crosslinking agent may be used to increase the strength of a productwhen the obtained thermally conductive composition is processed into asheet form or the like. As the crosslinking agents, crosslinking agentsto be thermally activated may also be used. Included in examples of thecrosslinking agent are lower alkoxylated aminoformaldehyde condensateswith 1-4 carbon atoms in the alkyl group, hexamethoxymethylmelamine (forexample, Cymell™ 303 by American Cyanamide), tetramethoxymethylurea (forexample, Beetle™ 65 by American Cyanamide) or tetrabutoxymethylurea(Beetle™ 85). Other useful crosslinking agents include polyfunctionalacrylates such as 1,6-hexanediol diacrylate and tripropyleneglycoldiacrylate. The crosslinking agent will usually be used in an amount of0.001-5 parts by weight to 100 parts by weight of the monomer.Combinations of the foregoing crosslinking agents may also be used.

A polymerizing mixture (thermally conductive composition precursor) isformed by combining a thickened binder component precursor comprisingthe (meth)acrylic monomer or a partially polymerized polymer obtained bypartial polymerization of the (meth)acrylic monomer, or a mixture of theaforementioned monomer and partially polymerized polymer, and apolymerization initiator, with a thermally conductive filler, andoptional crosslinking agents, chain transfer agents or additives. Any ofthe polymerization methods used for producing the aforementionedpartially polymerized polymers may be used. However, thermalpolymerization or ultraviolet polymerization is commonly used. The samethermal polymerization initiators or photopolymerization initiators asdescribed in the above partially polymerized polymers may be used forthis polymerization. Moreover, two or more polymerization initiatorswith different half-lives may also be used to form the polymerizingmixture.

When the binder component precursor is to be polymerized by thermalpolymerization, the thermally conductive composition of the presentinvention can be produced by the following procedure. The thermallyconductive composition precursor is deaired and mixed by a planetarymixer or the like. The resulting polymerizing mixture can be used as athermally conductive adhesive by filling the mixture in a liquid statein the location or area between the portions to be adhered and thenthermally polymerizing at 50-200° C. Alternatively, the polymerizingmixture is heated at about 50-200° C. for thermal polymerizationreaction to give a thermally conductive composition according to theinvention. As a (meth)acrylic monomer, a (meth)acrylic monomer havingany of acidic, neutral or basic nature in a molecule can be used.Further, a thermally conductive filler having any of acidic, neutral orbasic nature can be used. A (meth)acrylic monomer and a thermallyconductive filler, when used together, can have either the same nature,or different nature. However, in the thermal polymerization reaction ofan acrylic monomer with a peroxide, the presence of reducing metal ionssometimes causes a promotion reaction termed a redox reaction. Attentionmust therefore be paid when an acidic (meth)acrylic monomer such asacrylic acid is used. That is, a reducing metal ion eluted from a metalmixer, a thermally conductive filler and electromagnetic wave-absorbingfiller may cause a redox reaction. This problem can be avoided bylowering a viscosity of the thermally polymerizable binder component byreducing a proportion of the partially polymerized polymer therein.Alternatively, the problem can be avoided by using a mixer made ofnonmetallic material or a mixer coated with a resin on its metalsurface.

When the binder component precursor is polymerized by ultravioletpolymerization, the thermally conductive composition of the presentinvention can be produced by the following procedure. The thermallyconductive composition precursor is deaerated and mixed with a planetarymixer or the like. The polymerizing mixture thus obtained is molded, andexposed to ultraviolet rays to give the thermally conductive compositionprovided that the molded body thickness is restricted to ensure anultraviolet ray transmittance for polymerization. The content of thethermally conductive filler is also restricted for the same reason.

When the thermally conductive sheet of the invention is prepared bypolymerization, the polymerization is preferably carried out afterapplying the composition to the surface of a support such as a liner orcoating the surface therewith and forming a sheet by calendering orpress molding. As a result, the thermally conductive sheet according tothe invention can be obtained. The sheet may be formed in an inertatmosphere of nitrogen or the like in order to prevent inhibition ofpolymerization by oxygen.

Further, the acrylic-based thermally conductive composition may beproduced by adding the thermally conductive filler to a solution of thebinder component in a suitable solvent such as ethyl acetate, andheating the contents to remove the solvent while the contents are beingsufficiently mixed to effect uniform dispersion of the filler.

When the thermally conductive sheet of the present invention is to beprepared, it can be obtained by applying the molten body or solution tothe surface of a support such as a liner or coating the surfacetherewith, forming a sheet by calendering or press molding and coolingthe sheet to solidify or by removing the solvent from the solution bydrying.

Applications

The acrylic-based thermally conductive composition and the thermallyconductive sheet of the present invention may be used for adhesion ofheat sinks or heat radiators to electronic parts, and particularlysemiconductor/electronic parts such as power transistors, graphic IC,chip sets, memory chips, central processing units (CPUs) and the like. Athickness of the sheets is mainly determined by considering a thermalresistance of the portions to be applied. Usually, the sheets preferablyhave a thickness of 5 mm or less because the thermal resistance becomessmall. However, when filled into a gap between a larger heat generatingpart and a heat dissipating part, or applied to conform toirregularities of a part surface, the sheets having a thickness greaterthan 5 mm may be suitable. When the sheets having a thickness greaterthan 5 mm are suitable, the thickness of the sheets is preferably lessthan 10 mm.

The thermally conductive sheet is provided by forming a thermallyconductive composition layer on a support or base which is releasable orrelease treated with respect to the thermally conductive composition. Inthis case, release of the support or base from the sheet during servicewill allow the latter to serve as a free-standing film. Alternatively,the thermally conductive sheet may also be used while anchored to thesupport or base for improved sheet strength. Polymer films are typicalas supports or bases, and for example there may be used films ofpolyethylene, polypropylene, polyimide, polyethylene terephthalate,polyethylene naphthalate, polytetrafluoroethylene, polyether ketone,polyethersulfone, polymethylterpene, polyetherimide, polysulfone,polyphenylene sulfide, polyamidoimide, polyesterimide and aromaticamides. When the heat resistance is particularly required, a polyimidefilm or polyamidoimide film is preferred. Moreover, the thermalconductivity may also be increased by adding a thermally conductivefiller to the support or base. Furthermore, as supports or bases theremay be mentioned metal foils of aluminum, copper or the like, or wovenfabrics, nonwoven fabrics or scrims formed from glass fibers, carbonfibers, nylon fibers or polyester fibers, or such fibers that have beenprepared by coating these fibers with a metal. The support or base maybe present on one or both surfaces of the sheet, or it may be embeddedin the sheet.

EXAMPLES Examples 1-11 and Comparative Examples 1-2

1. Production of Partially Polymerized Polymer (Partially PolymerizedPolymer 1-2)

An ultraviolet polymerization initiator (Irgacure™ 651(2,2-dimethoxy-1,2-diphenylethan-1-one), product of Ciba-SpecialtyChemical) in an amount of 0.04 part by weight was mixed with 100 partsby weight of 2-ethylhexyl acrylate (2-EHA), and the mixture was exposedto ultraviolet rays at an intensity of 3 mW/cm² using an ultravioletlight source having a maximum intensity at wavelength of 300-400 nm togive partially polymerized polymer 1 with a viscosity of approximately1000 centipoise (cP).

An ultraviolet polymerization initiator (Irgacure™ 651) in an amount of0.04 part by weight and 2-ethylhexyl thioglycolate as a chain transferagent in an amount of 0.4 parts by weight were mixed with 100 parts byweight of 2-ethylhexyl acrylate (2-EHA), and the mixture was exposed toultraviolet rays at an intensity of 3 mW/cm² using an ultraviolet lightsource having a maximum intensity at wave length of 300-400 nm to givepartially polymerized polymer 2 with a viscosity of approximately 1000centipoise (cP).

2. Fabrication of Thermally Conductive Composition

Thermally conductive composition precursors obtained by deaerating andkneading the components listed in Table 1 below by a mixer were eachsandwiched between two polyethylene terephthalate (PET) liners coatedwith a silicone releasing agent and subjected to calendering. Themolding was followed by heating in an oven at 150° C. for 15 minutes forthermal polymerization, to give thermally conductive sheets having athickness of 1 mm (Examples 1 to 11, and Comparative Example 1).

In Comparative Example 2, the components shown in Table 1 were mixed attemperature of the melting point of the wax (80° C.) or higher. Themixtures were sandwiched between two PET liners, and subjected tocalendaring on a hot coater heated to 80° C. to give thermallyconductive sheets having a thickness of 1 mm.

In Example 1, the binder component was composed of an 80/20 (weightratio) copolymer of octadecyl methacrylate (ODMA) (crystalline acrylicmonomer) and 2-ethylhexyl acrylate (noncrystalline acrylic monomer).

In Example 2, the binder component was composed of a 90/10 (weightratio) ODMA and 2-ethylhexyl acrylate.

In Example 3, the binder component was composed of a 90/10 (weightratio) ODMA and 2-ethylhexyl acrylate, and the copolymer was crosslinkedwith hexanediol diacrylate.

In Example 4, the binder component was composed of an 80/20 (weightratio) copolymer of octadecyl acrylate (ODA) (crystalline acrylicmonomer) and 2-ethylhexyl acrylate (noncrystalline acrylic monomer).

In Example 5, the binder component was composed of a 50/50 (weightratio) copolymer of a C₁₈-C₂₄ alkyl methacrylate mixture (crystallineacrylic monomer) containing behenyl methacrylate (VMA) as a majorcomponent and 2-ethylhexyl acrylate.

In Example 6, the binder component was composed of a 55/45 (weightratio) copolymer of a C₁₈-C₂₄ alkyl methacrylate mixture (crystallineacrylic monomer) containing behenyl methacrylate (VMA) as a majorcomponent and 2-ethylhexyl acrylate (noncrystalline acrylic monomer).

In Example 7, the binder component was composed of an 80/20 (weightratio) mixture of (a) a 69/31 (weight ratio) copolymer of a C₁₈-C₂₄alkyl methacrylate mixture (crystalline acrylic monomer) containingbehenyl methacrylate (VMA) as a major component and 2-ethylhexylacrylate (noncrystalline acrylic monomer) and (b) a low molecular weightpolyacrylate (noncrystalline acrylic polymer).

In Example 8, the binder component was composed of an 80/20 (weightratio) mixture of (a) a 69/31 (weight ratio) copolymer of a C₁₈-C₂₄alkyl methacrylate mixture (crystalline acrylic monomer) containingbehenyl methacrylate (VMA) as a major component and 2-ethylhexylacrylate (noncrystalline acrylic monomer) and (b) a low molecular weightpolyacrylate (noncrystalline acrylic polymer).

In Example 9, the binder component was composed of an 80/20 (weightratio) mixture of an octadecyl methacrylate (ODMA) homopolymer(crystalline acrylic polymer) and a low molecular weight polyacrylate(noncrystalline acrylic polymer).

In Example 10, the binder component was composed of a 40/60 (weightratio) mixture of a poly(octadecyl acrylate (ODA)) (crystalline acrylicpolymer) and a 2-ethyl hexyl acrylate homopolymer (noncrystallineacrylic polymer).

In Example 11, the binder component was composed of a 25/75 (weightratio) mixture of a poly(octadecyl acrylate (ODA)) (crystalline acrylicpolymer) and a 2-ethyl hexyl acrylate homopolymer (noncrystallineacrylic polymer).

In Comparative Example 1, a 2-ethylhexyl acrylate homopolymer was usedas the binder component. Further, in Comparative Example 2, paraffin waxand a polyisobutylene were used as the binder component.

3. Measurement of Melting Point

Using a differential scanning calorimeter (product of Perkin Elmer), asample was heated at a rate of 5° C./min and the endothermic peaktemperature is defined as the melting point. The results are shown inTable 1 below.

4. Measurement of Thermal Resistance

After a cut sample in size of 10×11 mm was sandwiched between a heatingelement and a cooling plate, a difference in temperature between theheating element and the cooling plate was measured when a constant loadof 6.9 N/cm² and an electric power of 12.7 W were applied and thethermal resistance was determined by the following equation.Thermal resistance(degC·cm²/W)=difference in temperature(degC)×area(cm²)/electric power(W)

The results are shown in Table 1.

5. Releasability of Sheet after Service

After measuring the above thermal conductivity, the releasability of asample from the heating element was examined when the sample temperaturerecovered to room temperature. The heating element was grasped with onehand, and the cooling plate was grasped with the other hand. Shear forcewas applied to the heating element and the cooling plate with both handsby twisting, and whether or not the heating element could be removed wasexamined. The evaluation criteria are as follows: o=the heating elementcan be released by light twisting; and x=the heating element cannot bereleased. The results are shown in Table 1. TABLE 1 Example Comp. Ex. 12 3 4 5 6 7 8 9 10 11 1 2 Binder component (wt. parts) Partially 20 1010 — 50 45 25 — — 20 30 65 — polymerized polymer 1 Partially — — — 20 —— — 25 — — — — — polymerized polymer 2 ODA — — — 80 — — — — — — — — —ODMA 80 90 90 — — — — — 80 — — — — VMA-70 — — — — 50 55 55 55 — — — — —2-EHA — — — — — — — — — 40 45 35 — UP1000 — — — — — — 20 20 20 — — — —Poly(ODA) — — — — — — — — — 40 25 — — Paraffin wax — — — — — — — — — — —— 75 Polyisobutylene — — — — — — — — — — — — 25 HDDA — — 0.2 — — — — — —— — 0.15 — Irganox 1076 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3— LPO 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 0.25 — BPTC0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 — Thermally conductivecomposition (volume parts) Binder component 40 40 40 40 40 40 40 40 4040 40 40 40 mentioned above Total filler 60 60 60 60 60 60 60 60 60 6060 60 60 amount Silicon carbide 40 40 40 40 40 40 40 40 40 40 40 40 40Aluminum 20 20 20 20 20 20 20 20 20 20 20 20 20 hydroxide Melting point28 32 29 43 33 36 40 45 35 41 41 — 53 (deg C.) Thermal 3.98 4.23 5.424.08 4.99 4.54 3.26 3.71 2.32 3.74 4.42 5.55 4.43 resistance afterservice (deg C. · cm²/W) Releasability after ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ ∘ xservice

The results are in the above table wherein ODA is octadecyl acrylate,ODMA is octadecyl methacrylate, VMA-70 is a mixture of alkylmethacrylates of 18 to 24 carbons having behenyl methacrylate with 22carbons as the major component (content of 70%) (product of Nihon YushiCo., Ltd.), 2-EHA is 2-ethylhexyl acrylate, UP-1000 is liquid lowmolecular weight polyacrylate (noncrystalline acrylic polymer) having amolecular weight of 3000 and a Tg of −55° C. (product of TOAGOSEI Co.,Ltd.), poly(ODA) is a polyoctadecyl acrylate (product of ScientificPolymer Products), the paraffin wax has a melting point of 56° C.,polyisobutylene has a molecular weight of 4000, HDDA is 1,6-hexanedioldiacrylate, Irganox 1076 is an antioxidant (product of Ciba-SpecialtyChemical), LPO is lauroyl peroxide, and BPTC is1,1-bis(t-butylperoxy)-3,3,5-trimethylcyclohexane, the silicon carbidehad an average particle size of 70 μm, and the aluminum hydroxide had anaverage particle size of 2 μM and was titanate-treated.

From Table 1, one can conclude that the thermally conductive sheets ofthe present invention in Examples 1 to 11 showed low thermal resistancesand good releasability after service. On the other hand, the thermallyconductive sheet in Comparative Example 1 in which an acrylic polymercontaining no crystalline acrylic polymer was used as a binder componentshowed a high thermal resistance. Moreover, the thermal conductive sheetin Comparative Example 2 in which wax was used as a binder componentcould not be released after service.

Examples 12 and Comparative Example 3

Preparation of Thermally Conductive Compositions and Measurement ofPhysical Properties

A thermally conductive composition precursor obtained by deaerating andkneading the component with a composition listed in Table 2 below by amixer was sandwiched between two PET liners coated with a siliconereleasing agent and subjected to calendering. The molding was followedby exposing the calendered sheet to ultraviolet light at an intensity of3 mW/cm² for 5 minutes for ultraviolet polymerization to give athermally conductive sheet having a thickness of 0.12 mm (Example 12).

In Comparative Example 3, the component shown in Table 2 was mixed attemperatures of the melting point of the wax (90° C.) or higher. Themixture was sandwiched between two PET liners, and subjected tocalendaring on a hot coater heated to 90° C. to give a thermallyconductive sheet having a thickness of 0.12 mm.

Moreover, measurements of melting points, thermal resistances andreleasability after service were carried out in the same manner asdescribed above. TABLE 2 Comparative Example 12 Example 3 Bindercomponent (wt. Parts) Partially polymerized polymer 1 40 — 2-EHA — — ODA60 — Irgacure 651 (ultraviolet 0.2 — polymerization initiator) Paraffinwax — 75 Polyisobutylene — 25 Thermally conductive composition (vol.parts) Binder component mentioned above 85 85 Boron nitride 15 15Melting point (deg C.) 29 53 Thermal resistance (deg C. · cm²/W) 2.282.68 Releasability after service ◯ XIrgacure 651 (product of Ciba-Specialty Chemical) was used as anultraviolet polymerization initiator. Boron nitride had an averageparticle size of 10 μm. The other components are the same as in Table 1.

From Table 2, one can conclude that the thermally conductive sheet ofthe present invention in Example 12 showed a low thermal resistance andgood releasability after service. On the other hand, the thermallyconductive sheet in Comparative Example 3 in which wax was used as abinder component could not be released after service.

The acrylic-based thermally conductive composition and the thermallyconductive sheet of the present invention show high thermalconductivities during service and adhesion to bodies with which they arecontacted are enhanced, and they exhibit good releasability afterservice.

1-6. (canceled)
 7. An acrylic-based thermally conductive compositioncomprising a binder component containing a crystalline acrylic polymerwith an alkyl group of 18 carbons or more and a thermally conductivefiller.
 8. A composition according to claim 7, wherein said crystallineacrylic polymer has a melting point of 25° C. or higher and 100° C. orlower.
 9. A composition according to claim 7, wherein said crystallineacrylic polymer is a polymer of a (meth)acrylate ester monomer with analkyl group of 18 carbons or more.
 10. A composition according to claim9, wherein said crystalline acrylic polymer is a copolymer of a(meth)acrylate ester monomer with an alkyl group of 18 carbons or moreand a noncrystalline acrylic monomer.
 11. A composition according toclaim 7, wherein said binder component is a mixture of the crystallineacrylic polymer and the noncrystalline acrylic polymer.
 12. Acomposition according to claim 8, wherein said binder component is amixture of the crystalline acrylic polymer and the noncrystallineacrylic polymer
 13. A composition according to claim 9, wherein saidbinder component is a mixture of the crystalline acrylic polymer and thenoncrystalline acrylic polymer
 14. A composition according to claim 10,wherein said binder component is a mixture of the crystalline acrylicpolymer and the noncrystalline acrylic polymer
 15. A compositionaccording to claim 8, wherein said crystalline acrylic polymer is apolymer of a (meth)acrylate ester monomer with an alkyl group of 18carbons or more.
 16. A composition according to claim 15, wherein saidcrystalline acrylic polymer is a copolymer of a (meth)acrylate estermonomer with an alkyl group of 18 carbons or more and a noncrystallineacrylic monomer.
 17. An acrylic-based thermally conductive sheetobtained by forming a composition according to claim 7 into a sheet. 18.An acrylic-based thermally conductive sheet obtained by forming acomposition according to claim 8 into a sheet.
 19. An acrylic-basedthermally conductive sheet obtained by forming a composition accordingto claim 9 into a sheet.
 20. An acrylic-based thermally conductive sheetobtained by forming a composition according to claim 10 into a sheet.21. An acrylic-based thermally conductive sheet obtained by forming acomposition according to claim 11 into a sheet.
 22. An acrylic-basedthermally conductive sheet obtained by forming a composition accordingto claim 12 into a sheet.
 23. An acrylic-based thermally conductivesheet obtained by forming a composition according to claim 13 into asheet.
 24. An acrylic-based thermally conductive sheet obtained byforming a composition according to claim 14 into a sheet.
 25. Anacrylic-based thermally conductive sheet obtained by forming acomposition according to claim 15 into a sheet.
 26. An acrylic-basedthermally conductive sheet obtained by forming a composition accordingto claim 16 into a sheet.